A Ring of Dark Matter

byPaul GilsteronMay 15, 2007

Dark matter has to be made up of some sort of elementary particle, but we know astoundingly little about it. Its existence can be inferred from its necessary effects — something we can ‘t see seems to be holding galaxy clusters together, because the gravity from the stars we do observe in them isn’t sufficient to do the job. That makes gathering any evidence for dark matter’s behavior — indeed, for its very existence — a crucial goal for astrophysicists. And today we have the strongest supporting evidence yet that dark matter is real.

The work comes via the Hubble Space Telescope, used by a team of astronomers to locate what appears to be a ring of dark matter in the cluster ZwCl0024+1652, some five billion light years from our Solar system. The ring is 2.6 million light years across, and this detection appears to be unique. Says M. James Jee (Johns Hopkins): “This is the first time we have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster.” Lee was a member of the team that found the dark matter ring.

Image: The galaxy cluster Cl 0024+17 (ZwCl0024+1652) as seen by Hubble’s Advanced Camera for Surveys. The image displays faint faraway background galaxies that had their light bent by the cluster’s strong gravitational field. By mapping the distorted light and using it to deduce how dark matter is distributed in the cluster, astronomers spotted the ring of dark matter. One of the background galaxies is located about two times further away than the yellow cluster galaxies in the foreground, and has been multiple-imaged into five separate arc-shaped components, seen in blue. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

A possible cause for the ring: The collision between two galaxy clusters some 1 to 2 billion years ago. Using earlier evidence of such a collision, the team’s computer simulations modeled the event, showing how any associated dark matter might react. In a way, says Holland Ford (also of Johns Hopkins), “Nature is doing an experiment for us that we can’t do in a lab, and it agrees with our theoretical models.”

Let’s be clear on the method here. Dark matter, by its nature, cannot be seen. Astronomers infer its existence by observing how its gravity bends the light of distant background galaxies. We’ve looked at that phenomenon, called gravitational lensing, before. It’s a powerful tool not only in the study of cosmological structure but is also helpful in exoplanet detections. Such lensing is a major way to learn about dark matter, but in this case the substance — whatever it is — is widely separated from the gas and galaxies that make up the clusters themselves.

That gives astronomers a chance to home in on qualities particular to dark matter, the things that distinguish it from the ordinary matter — stars, planets, people — that make up the 4 percent of the universe we can actually see. And what has shown up is an unexpected ‘rippling’ effect that caused ripples of its own among team members:

“I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” Jee explained. “I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.”

Jee goes on to explain the ripple effect:

“The collision between the two galaxy clusters created a ripple of dark matter which left distinct footprints in the shapes of the background galaxies. It’s like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.”

So now we’re seeing dark matter in a new kind of distribution, offering further clues about its nature. I sometimes think of dark matter (and the equally mysterious dark energy) as needed correctives to the notion that we are within a few years (or decades, perhaps) of truly understanding the cosmos. That point of view, which settles in every few centuries only to be disrupted by new scientific breakthroughs, now needs the adjustment supplied by a simple fact: We have no good explanation for more than a fraction of the matter that pervades the universe, nor do we fathom the relentless acceleration now thought to fuel its expansion.

An excellent video on these findings is available here. Younger readers who are contemplating a career in astrophysics should rush to get into work like this. We are entering an era of unprecedented discovery.

Comments on this entry are closed.

ljkMay 15, 2007, 17:13

The tension of cosmological magnetic fields as a contribution to dark energy

Abstract: We propose that cosmological magnetic fields generated in regions of finite spatial dimensions may manifest themselves in the global dynamics of the Universe as `dark energy’. We test our model in the context of spatially flat cosmological models by assuming that the Universe contains non-relativistic matter $\rho_m\propto \alpha^{-3}$, dark energy $\rho_{Q}\propto \alpha^{-3(1+w)}$, and an extra fluid with $\rho_{B} \propto \alpha^{n-3}$ that corresponds to the magnetic field. We place constraints on the main cosmological parameters of our model by combining the recent supernovae type Ia data and the differential ages of passively evolving galaxies. In particular, we find that the model which best reproduces the observational data when $\Omega_m=0.26$ is one with $\Omega_{B}\simeq 0.03$, $n\simeq 7.68$, $\Omega_{Q}\simeq 0.71$ and $w\simeq -0.8$.

Title: $\Lambda$CDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?

Authors: Richard Lieu

(Submitted on 17 May 2007)

Abstract: Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter {\it and now} dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of $\Lambda$CDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind $\Lambda$CDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark.

$\Lambda$CDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?

Authors: Richard Lieu

(Submitted on 17 May 2007)

Abstract: Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter {\it and now} dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of $\Lambda$CDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind $\Lambda$CDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark.

Abstract: This Resource Letter provides a guide to the literature on dark energy and the accelerating universe. It is intended to be of use to researchers, teachers, and students at several levels. Journal articles, books, and websites are cited for the following topics: Einstein’s cosmological constant, quintessence or dynamical scalar fields, modified cosmic gravity, relations to high energy physics, cosmological probes and observations, terrestrial probes, calculational tools and parameter estimation, teaching strategies and educational resources, and the fate of the universe.

Abstract: While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.

Abstract: This is a short review, aimed at a general audience, of several current subjects of research in cosmology. The topics discussed include the cosmic microwave background (CMB), with particular emphasis on its relevance for testing inflation; dark matter, with a brief review of astrophysical evidence and more emphasis on particle candidates; and cosmic acceleration and some of the ideas that have been put forward to explain it. A glossary of technical terms and acronyms is provided.

Comments: Submitted for publication in “Visions of Discovery” (in honor of Charles Townes), to be published by Cambridge University Press. 53 pages, 13 figures

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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